Early studies proposed and demonstrated that the radiation pressure that light exerts on matter can be used to manipulate particles ranging in size from single atoms to large cells using laser light A. Ashkin, Proc. Natl. Acad. Sci. USA, 94: 4853, 1997. Since then, a number of methods to control the location of (trap) particles based on this idea have been developed. These include both dual beam traps, and single beam traps such as levitation traps and optical tweezers. The dual beam trap shown in FIG. 1a is capable of trapping a particle at the equilibrium point E using two Gaussian laser beams, i.e., it can be confined in x-, y-, and z-directions simultaneously. The confinement along x and y results from a gradient force due to the decreasing intensity of the laser beam away from the optical axis through points A, B, and E. Trapping along z is accomplished by balancing the scattering force exerted on the particle by the two counterpropagating beams at point E. U.S. Pat. Nos. 7,127,146; 7,149,396; 3,710,279; Constable et al., Opt. Lett. 18: 1867, 1993; Cran-McGreehin et al., Lab on a Chip 6: 1122, 2006; Proc. Natl. Acad. Sci. USA, 94: 4853, 1997; M. J. Renn et al., Phys. Rev. Lett., 82: 1574, 1999.
The scattering force Fs is proportional to Fs˜P/A, where P is the total power contained in the beam and A is the beam area. The conventional dual beam trap of FIG. 1a works based on the difference of beam area AL(z) and AR(z) between the left and right propagating beams as a function of z. At point E where both beam areas are the same, there is no net scattering force. The particle is trapped because any displacement from point E along z will create an imbalance between FSL and FSR that drives the particle back towards E.
A need exists in the art to be able to trap particles inside microfluidic channels without having to use external microscopes to create the required beam profiles that are used in the dual beam trap or laser tweezers. In particular, a preferred solution would be to accomplish trapping with integrated optical waveguides. Unfortunately, the conventional dual beam trap as described above cannot be implemented in a waveguide by tailoring its shape. The reason is that two counterpropagating beams in a waveguide will always be confined to the same mode area (such as the one shown in FIG. 1b) at any given point along the propagation direction. Therefore, there is no way to create the imbalance in area that would result in a z-dependent scattering force as in the dual beam trap.